Tips to kill gnar?

I played a couple ranked games today and one of them was top vs gnar. I main morde and did fine though we both went 0-0-0 in lane. He's too mobile to kill in normal form, even with rylais and boots with my Ulti perma slow. And he's also to strong to kill in mega form. I came close by baiting him to fight in mega form but I still couldn't kill him with a full rotation of abilities and ignite. When exactly is the window to kill this guy? Also gj riot making a champ as annoying as teemo, zyra, or heimer to lane against.
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